The primary focus in the development of service robotic systems is still mostly on purely functional concerns (what to do?). However, an important further aspect so far often neglected are the non-functional properties (NFPs) of task execution (how to do it?). In this paper, we briefly introduce our general approach for composing NFPs and for deciding about adequate execution alternatives. Then, we present a model for the time composition of an order picking task. Finally, we show in a practical experiment how a service robot executing an order picking task can balance the fulfilment of external nonfunctional requirements (meet deadline for order picking) and the consideration of internal needs (save energy).

Localization and Mapping are the mandatory functionalities of an autonomous mobile robot. These functionalities are implemented mostly using expensive laser range sensors which are characterized by long range, wide opening angle and precision. Recent developments in low-cost RGBD cameras make them suitable for robot mapping and localization. However, limitations like viewing angle, range and accuracy prevent them from replacing laser rangers in large areas. With such RGBD cameras, mapping large areas is hard but localization in large areas can be achieved by augmenting them with additional information. This makes the low-cost RGBD cameras suitable for a cost-effective localization solution, especially for the fleets of robots operating in large areas, where one robot equipped with a laser ranger is used in mapping while the other robots equipped with low-cost RGBD cameras for localization. Adaptive Monte Carlo Localization(AMCL) is the most widely used algorithm for mobile robot localization. Our proposed solution extends the AMCL to use a virtual laser generated from RGBD camera and visual markers to solve the issues concerning the global localization, reducing the uncertainty in pose estimates during tracking and kidnapped robot problem. Our approach is tested in large areas and the uncertainty during the localization is compared against the result from using a laser ranger.

We consider a signaling format where the information to be communicated from a single or multiple transmitters to a receiver is modulated via a superposition of independent data streams. Each data stream is formed by error-correction encoding, constellation mapping, replication and permutation of symbols, and application of signature sequences. The relations between the data bits and modulation symbols transmitted over the channel can be represented by a sparse graph. In the case where the modulated data streams are transmitted with time offsets the receiver observes spatial coupling of the individual graphs into a graph chain enabling efficient demodulation/decoding. We prove that a two-stage demodulation/decoding method, in which iterative demodulation based on symbol estimation and interference cancellation is followed by parallel error-correction decoding, achieves capacity on the additive white Gaussian noise channel asymptotically. We compare the performance of the two-stage receiver to the receiver which utilizes hard-decision decoding feedback between the error-correction decoders and the iterative demodulator and estimate the gap between the achievable spectral efficiency and the channel capacity.

The high system flexibility necessary for the full automation of complex and unstructured tasks leads to increased technological complexity, thus to higher costs and lower performance. In this paper, after an introduction to the different dimensions of flexibility, a method for flexible modular configuration and evaluation of systems of systems is introduced. The method starts from process requirements and, considering factors such as feasibility, development costs, market potential and effective impact on the current processes, enables the evaluation of a flexible systems of systems equipped with the needed functionalities before its actual development. This allows setting the focus on those aspects of flexibility that add market value to the system, thus promoting the efficient development of systems addressed to interested customers in intralogistics. An example of application of the method is given and discussed.

Large-scale (a.k.a. Massive) Multiple-Input Multiple-Output (MIMO) systems are considered as a strong candidate to meet the exceptionally high spectral efficiency requirements for “beyond 4G” (or commonly termed 5G) wireless communications systems. For such systems, the availability of accurate uplink channel knowledge at the base station is critical to success, particularly in the time-division duplex (TDD) mode, where channel reciprocity is exploited to employ efficient downlink beamforming/precoding schemes. A major obstacle to acquiring such channel knowledge at the base station, however, is posed by the potential uplink pilot interference in multi-cell environments known as pilot contamination. In a recent contribution [1], it is shown that pilot contamination can be sidestepped with the aid of a simple interference management scheduling protocol. Building and expanding on [1], we elaborate on the design of shift-and mutually orthogonal pilots, and propose a multi-band operation to expand the number of users to be serviced in densely-populated areas. More specifically, the proposed design adjusts the transmission bandwidth to exploit the spatio-temporal resolution properties of wideband wireless channels, which, combined with the shift-orthogonality principle, enable to allocate identical frequency resources to a number of closely-spaced users. In addition, the pilot transmission technique presented herein minimizes guard interval overhead in the OFDM context, and can be realized with quasi-constant envelope, maximizing battery efficiency in user handsets.

Fast re-organization of a production facility and its production flows enables the automated and efficient production of individualized products up to a lot size of one single instance in large-scale. Flexible service robots replacing hard wired transportation belts are one of the enablers of the Industry 4.0 idea. Within this paper we propose a distributed approach for autonomous robots forming a fleet capable of performing transportation tasks within an industrial production system. The system is able to adapt to changes in the production flow by changing the software configuration only. The presented approach combines the predictability and efficiency of path based navigation with the flexibility of fast reactive local obstacle avoidance. The combination enables reliable, predictable and robust navigation of many robots in a fleet, that are able to cope with the challenges of a dynamic environment such as obstacles blocking the way and persons moving around.

Mobile Agricultural Robot Swarms (MARS) is an approach for autonomous farming operations by a coordinated group of robots. One key aspect of the MARS concept is the low individual intelligence, meaning that each robot is equipped with only a minimum of sensor technology in order to achieve a low cost and energy efficient system that provides scalability and reliability for field tasks. The robot swarms are coordinated by a centralized entity (OptiVisor) which is responsible for path planning, optimization and supervision. It also serves as a mediator between the robots and different cloud services responsible for the documentation of the procedure. This paper focuses on the architecture and function of OptiVisor within the overall MARS system. An OptiVisor in combination with a simulation environment for a robot swarm is presented and shows the feasibility of the general concept and the current state of the algorithms. Furthermore, the paper shows results about the current progress of OptiVisor integration using real robots.

In this paper, a fast reactive obstacle avoidance approach for omnidirectional driving is presented. The method is based on the dynamic window approach, but uses a cuboid instead of a window to limit the 3-dimensional search space accordingly to the dynamic constraints of the robot. Besides the kinematics and dynamics, the presented approach also considers the shape of a robot. To cope with the effort of the time consuming distance calculations, the remaining distance values are precalculated in an offline part and provided by a lookup table. This procedure is based on the Curvature Distance Lookup (CDL) approach which is extensively used in several real world robotic applications but which was so far only implemented for 2-DOFs. During the online phase, the extended approach enables the selection of a motion command from a wide range of curvatures (3-DOFs) within the current dynamic cuboid. The distance values are queried from the corresponding lookup table entries depending on the occupancy grid determined by latest sensor information. The reduced computational effort of the control loop allows to consider obstacle information from corresponding sources to the full extent and without preprocessing. Furthermore, complex heuristics can be implemented to evaluate a high number of omni-drive velocity triples in such a way that the driving behavior of the robot is influenced accordingly.

Throughput-delay tradeoff of random access over satellite links is analyzed and scaling laws are derived for the cases of the collision channel and the multipacket reception (MPR) channel as well as repetition random access. It is shown that multiuser detection and repetition schemes improve the multiple access performance in the sense that the inevitable compromise between throughput and delay is mitigated by joint detection capabilities and/or repetitions.

A probabilistic medium-access control (MAC) protocol is proposed for an uncoordinated network of nodes with multipacket reception (MPR) capability at the receiver. The protocol uses binary feedback at the end of each time slot and uses an extended Kalman filter (EKF) to compute an estimate of the number of currently active nodes in the service area. The estimate is then used to optimize the expected system throughput by adjusting the medium access probability of each node. Simulations show that the proposed MAC protocol succeeds in tracking the number of nodes and achieving near-optimal throughput performance.

A novel modulation format based on coupling for streaming data transmissions is proposed. The construction utilizes an infinite parity-check matrix of a low-density parity-check convolutional code which is used for data encoding with real-domain addition instead of modulo-two addition. The demodulation and decoding is accomplished using iterative bit estimation and interference cancellation. We demonstrate that the threshold saturation effect of spatial graph coupling holds for the proposed format and the achievable communication rate approaches the additive white Gaussian noise channel capacity and exceeds the capacities of traditional PAM modulations.

Reliable object recognition is a mandatory prerequisite for Service Robots in everyday environments. Typical approaches for object recognition use single algorithms or features. However, none is yet able to classify across all types of objects and the field of object recognition is thus still an open challenge. We propose an approach for object recognition and pose estimation that combines existing algorithms. Probabilistic methods are used to fuse the classification and pose estimation results, considering the error introduced by the measurements, actuators (sensor on manipulator) and algorithms. Since integration is one of the real challenges from the laboratory towards the real world, we demonstrate the approach in two fully integrated scenarios. We run the experiments on two platforms and focus on the distinction of few but similar objects.

The threshold, or saturation phenomenon of spatially coupled systems is revisited in the light of Lyapunov's theory of dynamical systems. It is shown that an application of Lyapunov's direct method can be used to quantitatively describe the threshold phenomenon, prove convergence, and compute threshold values. This provides a general proof methodology for the various systems recently studied.

The error floor of LDPC is revisited as an effect of dynamic message behavior in the so-called absorbing sets of the code. It is shown that if the signal growth in the absorbing sets is properly balanced by the growth of set-external messages, the error floor can be lowered to essentially arbitrarily low levels. Importance sampling techniques are discussed and used to verify the analysis, as well as to discuss the impact of iterations and message quantization on the code performance in the ultra-low BER (error floor) regime.

Demodulation in a random multiple access channel is considered where the signals are chosen uniformly randomly with unit energy. It is shown that by lifting (replicating) the graph of this system and randomizing the graph connections, a simple iterative cancellation demodulator achieves the same performance as an optimal symbol-by-symbol detector of the original system. The iterative detector has a complexity that is linear in the number of users, while the direct optimal approach is known to be NP-hard. However, the maximal system load of this lifted graph is limited to$\alpha < 2.07$, even for large signal-to-noise ratios (SNRs)—the system is interference limited. Spatial coupling between subsequent lifted graphs is introduced, and anchoring the initial graphs, the achievable system load$\alpha$can go to infinity as the SNR goes to infinity. Our results apply to several well-documented system proposals, such as interleave-division multiple access, partitioned spreading, and certain forms of multiple-input multiple-output communications.

Multipacket reception (MPR) capability provides the opportunity to improve bandwidth utilization while reducing complexity of the medium access control layer. Specifically, it has been shown that ALOHA random access tends to become asymptotically optimal as the receiver's joint decoding capability of the receiver grows to infinity. However, the average traffic offered to the medium is required to remain below a threshold in order for the MPR capability to be utilized efficiently. The effect of controlling the access probability according to the instantaneous offered traffic is studied in this paper. It is shown that, as the average rate of the offered traffic exceeds the optimal value, a dynamic control strategy can increase the maximum expected throughput of probabilistic access dramatically compared to uncontrolled ALOHA. The average throughput achieved by this strategy is analyzed and a lower bound on the system throughput of genie-aided probabilistic access is derived. Finally, the effect of uncertainty about the offered traffic is addressed and an approximation for the resulting performance loss is presented.

The development of service robots has gained more and more attention over the last years. A major challenge on the way towards industrial-strength service robotic systems is to make the step from code-driven to model-driven engineering. In this work we propose to put models into the focus of the whole life-cycle of robotic systems covering design-time as well as run-time. We describe how to explicate parameters, properties and resource information in the models at design-time and how to take these information into account by the run-time system of the robot to support its decision making process. We underpin our work by an exhaustive real-world example which is completely developed with our tools.

Equalization for uplink MIMO SC-FDMA LTE multi-stream transmission is considered utilizing linear filtering methods and redundancy in the form of channel coding. The equalizer jointly removes inter-symbol (ISI) as well as spatial (MIMO) interference. It is observed that in an unequal received power situation, gains over traditional MMSE frequency equalization can be obtained via group-based cancellation with turbo equalization. It is shown that signal co-existence is possible with this methodology and therefore QoS for cell-edge users can be maintained at considerably higher noise levels as compared to a baseline receiver.

The development of service robots has gained more and more attention over the last years. Advanced robots have to cope with many different situations and contingencies while executing concurrent and interruptable complex tasks. In particular, mobile manipulation tasks increase the complexity. To manage the raising number of tasks and execution variants in complex environments there is a tremendous need for context and situation dependent composition and selection of reusable skills. This requires explicit descriptions of relevant properties and parameters of the robot, its resources and its capabilities. Different views on partial aspects of a robot system (mechanical, electrical and even software) can be provided by different models as is already common practice at design-time. However, these design-time models also need to be accessible at run-time to support run-time reasoning of the robot in order to adequately compose its skills and assign resources. That requires to extract useful information out of the design-time models and to transform it into representations which can be exploited at run-time. We present an approach to exploit information provided via design-time models (e.g. software components, simulation, planning) for run-time decision making. It allows for more informed decisions on how to compose action plots at run-time in order to manage the huge amount of different execution variants in service robotics.

The basic design challenges for large-scale modern error control decoders based on message passing are examined in this review and exploratory paper. Space, complexity, and power consumption figures are of most interest to the design engineer, and the state-of-the art of current implementations are presented. Fundamental limits of performance versus power and complexity are discussed, and innovative state-of-the art approaches to address these challenges are highlighted.

Demodulation in a random multiple access channel is considered where the signals are chosen uniformly randomly with unit energy, a model applicable to several modern transmission systems. It is shown that by lifting (replicating) the graph of this system and randomizing the graph connections, a simple iterative cancellation demodulator can be constructed which achieves the same performance as an optimal symbol-by-symbol detector of the original system. The iterative detector has a complexity that is linear in the number of signals (users), while the direct optimal approach is known to be NP-hard. However, the maximal system load of this lifted graph is limited to α <; 2:074, even for signal-to-noise ratios going to infinity - the system is interference limited. We then show that by introducing spatial coupling and anchoring of the lifted graph, this limitation can be avoided and arbitrary system loads are achievable. Our results apply to several well-documented system proposals, such as IDMA, partitioned spreading, and certain forms of MIMO communications.

Communication systems that perform iterations between equalizer and decoder at the receiver are known as iterative or turbo equalization systems. These iterative systems can achieve large performance gains (in terms of bit error rates) when compared to their non-iterative counterparts. A major issue in such schemes is that optimal equalization methods are too complex for practical implementation when dealing with large-memory channels and/or higher-order modulations. This problem has received a lot of attention in the specialized literature and several suboptimal low-complexity equalization methods have been proposed. One such method makes use of the jointly Gaussian approach for equalization and was shown to be equivalent to minimum mean-square error linear equalization. In this paper an extension of the jointly Gaussian approach for equalization, dubbed extended jointly Gaussian approach, is proposed. This extension improves the bit error rate performance of the system at the expense of a controllable increase in computational complexity. Signal-to-distortion analysis, EXIT charts and bit error rate curves are used to assess the performance of this new method.

Localization and mapping are fundamental problems in service robotics since representations of the environment and knowledge about the own pose significantly simplify the implementation of a series of high-level applications. ToF (time-of-flight) cameras are a relatively new kind of sensors in robotics. They enable the real-time capture of the distance and the grayscale information of a scene. Due to the increase of the image resolution of ToF cameras, now highlevel computer vision algorithms for visual feature extraction (e.g. SIFT or SURF) can be applied to the captured images. These visual features combined with the corresponding distance information give a full measurement of 3D landmarks. An obvious problem to be solved is the continuously growing number of landmarks. So far, all ever seen landmarks are just accumulated irrespective of their utility and the then required resources. Rather, one should keep only really useful landmarks, e.g. such that localization quality in the whole operational area is kept above a given threshold. In fact a lifelong running SLAM approach is dependent on means to select and discard landmarks. That is even more acute in case of feature-rich sensor data as provided with high update rates by sensors like a ToF camera. We run our SLAM approach in a real-world experiment within an indoor environment. The experiment was performed on a P3DX-platform equipped with a PMD CamCube 2.0 and a Xsens IMU.

Computationally efficient joint detection in CDMA packetized communication is considered. The joint detection is based on iterative cancellation and utilizes only low complexity individual data receivers. The savings in complexity compared to other alternatives proposed in the literature is due to an encoding scheme known as partition spreading that can be decoded using algorithms that are similar to well-known turbo and sum-product decoding. Besides having a low computational complexity, the technique offers near-far resistant performance and can achieve higher system loads than conventional CDMA. The low complexity of these component receivers allows a large number of users to be implemented onto a single FPGA. A Virtex-IV on a Lyrtech Development board is used to implement a test bed for this PS-CDMA system. The implementation focuses on area optimization to give 50 users in a single Virtex-IV with 84% slice utilization and a maximum aggregate throughput of 192Mb/s. The measured performance of this prototype is compared against theoretical results on PS-CDMA in environments with varying power levels. An FPGA resource-performance analysis is given.

Theoretical limits of performance of error control coding in iterative cancellation systems is explored. Limits of the modulation symbol estimation error attainable by any code are given. Required properties of optimal codes for cancellation are exposed, and the actual performance of several popular code families is discussed.

It is shown that dominant trapping sets of regular low-density parity-check (LDPC) codes, so-called absorption sets, undergo a two-phased dynamic behavior in the iterative message-passing decoding algorithm. Using a linear dynamic model for the iteration behavior of these sets, it is shown that they undergo an initial geometric growth phase which stabilizes in a final bit-flipping behavior where the algorithm reaches a fixed point. This analysis is shown to lead to very accurate numerical calculations of the error floor bit error rates down to error rates that are inaccessible by simulation. The topology of the dominant absorption sets of an example code, the IEEE 802.3an (2048,1723) regular LDPC code, are identified and tabulated using topological relationships in combination with search algorithms.

Emulation of fading channels is a key step in the design and verification of wireless communication systems. Testing wireless transceivers with actual fading channels is inconvenient due to unrepeatable and uncontrollable channel conditions. In this paper we present a compact field-programmable gate array (FPGA) implementation for a circuit that generates temporally-correlated fading variates for emulating multipath fading radio channels. The implemented fading emulator is flexible enough to model different propagation scenarios accurately and is compact enough that it can be implemented on the same FPGA with the design under test (DUT) for greater emulation efficiency and speed-up. Several streams of Rayleigh or Rician fading variates are generated by passing independent samples of Gaussian noise through spectrum shaping filters. The new baseband emulator is fully parameterizable and can emulate a wide variety of single and multiple antenna scenarios.

Medium access in ad-hoc wireless networks must be performed in a distributed fashion due to lack of coordination between nodes. Specifically, when nodes are capable of receiving more than one transmission simultaneously, the design of distributed medium-access mechanisms that efficiently exploit the receiver's capability becomes more challenging. Adaptive probabilistic medium access for ad-hoc wireless networks is proposed in this paper. Nodes with data packets to transmit perform an announcement process in order to inform other nodes of their intended traffic. The acquired information through this process about other potential transmitters in the vicinity is then used by the nodes to choose a transmission probability with which they transmit their data packets. The performance of a multi-packet reception capable ad-hoc wireless network under the proposed protocol is analyzed and evaluated numerically and via simulations, and compared with Aloha-type random access.

Wireless ad hoc networks with nodes capable of simultaneous multiple packet reception are considered. We focus on spread spectrum networks and address the relationship between the packet detection success, probability of the packet success over multiple hops, and asymptotic throughput capacity of the network in terms of power and bandwidth resources as well as the multi-packet detection capability of the nodes. In the second part of the paper we consider network with nodes employing partitioned code division multiple access (CDMA) transmission and joint iterative reception. We study local communication in the network and derive a relationship between the probability of detection success and a fraction of the multiple access channel capacity that can be achieved at any communicating node. We use this result to demonstrate that near optimum throughput and reliable end-to-end communication can be achieved in the network with use of a practical detection method. Finally, we present simulation results which demonstrate the advantage of partitioned CDMA with iterative receivers over CDMA with linear receivers in a network setting.

Acting in everyday-life environments is still a great challenge in service robotics. Although algorithms and solutions already exist for many relevant subproblems, in particular the aspect of robustness and suitability for everyday use has been neglected so far very often. Robustness and suitability for everyday use are features affecting not only the overall system design but have impact on each single algorithm of each component.

This article describes a significantly improved sum-of-sinusoids-based model for the accurate simulation of time-correlated Rayleigh and Rician fading channels. The proposed model utilises random walk processes instead of random variables for some of the sinusoid parameters to more accurately reproduce the behaviour of wireless radio propagation. Every fading block generated using our model has accurate statistical properties on its own and hence, unlike previously proposed models, there is no need for time-consuming ensemble-averaging over multiple blocks. Using numerical simulation it is shown that the important statistical properties of the generated fading samples have excellent agreement with the theoretical reference functions. A fixed-point hardware implementation of the corresponding Rayleigh and Rician fading channel simulator on a field-programmable gate array (FPGA) is presented. By efficiently scheduling the operations, the reconfigurable fading channel simulator is compact enough that it can be efficiently used to simulate multipath scenarios and multiple-antenna systems (e.g. a 4×4 MIMO channel) using a single FPGA.

Iterative processing for linear matrix channels, aka turbo equalization, turbo demodulation, or turbo code-division multiple access (CDMA), has traditionally been addressed as the concatenation of conventional error control codes with the linear (matrix) channel. However, in several situations, such as CDMA, multiple-input-multiple-output (MIMO) channels, orthogonal frequency-division multiplexing (OFDM), and intersymbol-interference (ISI) channels, the channel itself either contains inherent signal redundancy or such redundancy can readily be introduced at the transmitter. For such systems, iterative demodulation of the linear channel exploiting this redundancy using simple iterative cancellation demodulators, followed by conventional feedforward error control decoding, provides a low-complexity, but extremely efficient decoding alternative. This two-stage demodulator/decoder outperforms more complex turbo CDMA methods for equal power modes (users). Furthermore, it is shown that arbitrary numbers of modes can be supported if an unequal power distribution is adopted. These power distributions are nested, which means that additional modes can be added without disturbing an existing mode population. The main result shows that these nested power distributions enable the two-stage receiver to approach the Shannon capacity of the channel to within less than one bit for any signal-to-noise ratio (SNR).

A novel hypothesis-based preamble detection method for uncoordinated, high-density packet-based communication over an additive white Gaussian noise channel is proposed and analyzed. Received samples are observed over a window of length equal to that of the preamble and a metric is computed for each sample shift of the window. A metric exceeding a noise dependent pre-computed threshold flags the presence of a preamble. The preamble sequence consists of concatenated sections of spreading sequences whose length is at most the coherence time of the channel. These sections are then differentially combined. A differential correlation-based detection is employed to locate the boundaries of the preamble. A theoretical framework is developed to provide exact analytical solutions for missing and falsely detecting a preamble using matrix analysis of quadratic Gaussian statistics. Furthermore, the robustness of the proposed methodology in a two path channel is studied. The effects of frequency and timing offsets on the system performance is evaluated. Simulation results are presented to validate the analytical expressions. Additionally, a performance comparison of the proposed differential detection scheme with that of a noncoherent square-law detector is presented.

A key advantage of viewing communications in wireless networks as multiple access rather than a plurality of point-to-point transmissions, is its robustness towards multiple access interference. Concurrent packet transmissions are allowed to coexist thus deviating from the traditional view of enforcing collision-footprints around the transmitter-receiver pairs. What are the performance gains of employing channel access strategy based on a multiple access channel in a multihop wireless network? We consider a wireless multihop network, where nodes have a joint decoding capability to resolve up to K multiple concurrent packet transmissions from other nodes in their range. The basic assumptions are that the packet transmissions are asynchronous, i.e., nodes are completely uncoordinated, and that the packet transmission at each node is based on a probabilistic model. In this paper, we show that a simple random access strategy for communication over such channels offers significant gains in throughput while reducing latency in congested wireless networks. More precisely, we characterize the throughput performance gains through an exact analysis for the case of K=2 and also offer tight approximations for arbitrary K. Furthermore, we study the asymptotic throughput behavior and prove asymptotic optimality of random channel access over multiple access channel.

We present a fading model with a compact and fast hardware implementation suitable for correlated Rayleigh fading channel simulators. The proposed scheme is based on the sum-of-sinusoids model because of its flexibility and efficient mapping onto hardware. Using numerical simulation, it is shown that the statistical properties of the generated fading variates match the theoretical reference model. Since the cross-correlations between sequences of generated fading variates are small, this model can also be used to implement a time-correlated multiple-input multiple-output (MIMO) fading channel simulator on a single field-programmable gate array (FPGA). The MIMO channel simulator can also be extended to support spatial correlation between generated fading samples. An implementation of a spatiotemporally correlated (4, 4) MIMO channel simulator on a Xilinx Virtex-II Pro XC2VP100-6 FPGA uses 46% of the configurable slices, 30% of the dedicated multipliers, and 32% of the on-chip block memories while generating 4 times 201 million 2 times 16-bit complex-valued fading samples per second.

Most commonly-used pseudo-random number generators (PNGs) in computer systems are based on linear recurrence. These deterministic PNGs have fast and compact implementations, andean ensure very long periods. However, the points generated by linear PNGs in fact have a regular lattice structure and are thus not suit able for applications that rely on the assumption of uniformly distributed pseudo-random numbers (PNs). In this paper we propose and evaluate several fast and compact linear, non-linear, and hybrid PNGs for a field- programmable gate array (FPGA). The PNGs have excellent equidistribution properties and very small autocorrelations, and have very long repetition periods. The distribution and long-range correlation properties of the new generators are efficiently, and much more rapidly, estimated at hardware speeds using designed modules within the FPGA. The results of these statistical tests confirm that the combination of several linear PNGs or the combination of even one small non-linear PNG with a linear PNG significantly improves the statistical properties of the generated PNs.

We present an accurate model for compact implementations of Rayleigh and Rician fading channels. Verification of the proposed fading simulator is performed by comparing the simulated statistics with those of the ideal reference models. A parameterizable field-programmable gate array (FPGA) implementation of the channel simulator is presented. The design is readily scalable to support multipath fading channels and multiple-input multiple-output (MIMO) systems. A 16-path fading channel, providing either Rician or Rayleigh fading, uses 41% of the configurable slices, 33% of the dedicated multipliers, and 32% of the on-chip block memories of a Xilinx Virtex-II Pro XC2VP100-6 FPGA while generating over 200 million complex- valued fading coefficients per second.

A stochastic sum-of-sinusoids based simulation model is proposed for Rayleigh and Rician fading channels. The time-averaged statistical properties of the new model have been significantly improved compared to existing models. Verification of the proposed fading simulator is carried out by comparing its measured statistical properties with the properties of the ideal reference models. The simulator utilizes a time-overlapped implementation strategy to provide a compact design suitable for multiple antenna simulators. An implementation of the resulting Rician fading simulator on a Xilinx Virtex-II Pro XC2VP100- 6 FPGA uses only 2% of the configurable slices, 1% of the dedicated multipliers, and 2% of the on-chip block memories while generating 201 million 2 times 16-bit complex-valued fading samples per second. The scalable design of the fading channel simulator enables a straightforward implementation of multiple antenna channels and different diversity schemes.

A major hindrance to the adoption of orthogonalized space-division multiplexing (SDM) via block diagonalization (BD) in multi-user MIMO downlinks is the need for channel state information (CSI) feedback from user terminals. Another drawback is lower than optimal sum rates, theoretically achievable with dirty paper coding (DPC). While multi-user diversity could be leveraged via user selection to narrow the sum- rate performance gap, it requires the presence of very large user pools. To help raise the practical feasibility of BD-SDM, we propose a scheme that jointly reduces CSI feedback while approaching optimal DPC sum rates with smaller user-pool sizes. Additionally, BD-SDM offers the flexibility for spatial mode allocation to cater for individual transmission rate requirements. This presents a challenging resource allocation problem because mode selection at one terminal affects the rates achieved at all other terminals and in turn, the overall sum rate. The proposed scheme offers a systematic means for resource allocation, while minimizing rate loss at the overall- and individual levels. It represents a streamlined process that simultaneously reduces CSI feedback while achieving sum rate maximization, user selection and systematic rate-loss minimizing resource allocation.

A compact, fast, and accurate realization of a digital Gaussian variate generator (GVG) based on the Box-Muller algorithm is presented. The proposed GVG has a faster Gaussian sample generation rate and higher tail accuracy with a lower hardware cost than published designs. The GVG design can be readily configured to achieve arbitrary tail accuracy (i.e., with a proposed 16-bit datapath up to plusmn15 times the standard deviation sigma) with only small variations in hardware utilization, and without degrading the output sample rate. Polynomial curve fitting is utilized along with a hybrid (i.e., combination of logarithmic and uniform) segmentation and a scaling scheme to maintain accuracy. A typical instantiation of the proposed GVG occupies only 534 configurable slices, two on-chip block memories, and three dedicated multipliers of the Xilinx Virtex-II XC2V4000-6 field-programmable gate array (FPGA) and operates at 248 MHz, generating 496 million Gaussian variates (GVs) per second within a range of plusmn6.66sigma. To accurately achieve a range of plusmn9.4sigma, the GVG uses 852 configurable slices, three block memories, and three on-chip dedicated multipliers of the same FPGA while still operating at 248 MHz, generating 496 million GVs per second. The core area and performance of a GVG implemented in a 90-nm CMOS technology are also given. The statistical characteristics of the GVG are evaluated and confirmed using multiple standard statistical goodness-of-fit tests.

This paper presents a computationally-efficient design and implementation technique for fading channel simulators. Our fixed-point implementation of a Rayleigh fading channel simulator on a field-programmable gate array (FPGA) utilizes only 4% of the configurable slices, 19% of the dedicated multipliers, and 2% of the on-chip memory blocks, while generating 12.5 million statistically accurate fading variates per second. The designed channel emulator can be parameterized to simulate a wide variety of different channel characteristics over bandwidths of up to 12.5 MHz. The compact simulator can also be instantiated multiple times to assess the performance of communication systems over multiple-input multiple-output (MIMO) channels.

A flexible and compact general-purpose filter processor is presented. This processor is intended for the hardware-based simulation of wireless channels on field-programmable gate arrays (FPGAs). When implemented on a Xilinx Virtex2P XC2VP100-6 FPGA, it utilizes 2% of the configurable slices, 9% of the dedicated 18times18-bitmultipliers, and 14 BlockRAMs. When paired with multiplicationfree interpolators, it can generate up to 300 million fading samples per second. The statistical properties of generated fading samples are shown to closely match the theoretical reference statistical properties.

A fast and compact implementation of nonlinear pseudo-random number generators (PNGs) on a field-programmable gate array (FPGA) is presented. The distribution and correlation properties of PNGs are efficiently, and very rapidly, estimated within the FPGA. It is shown that the pseudo-random output of nonlinear PNGs have significantly improved randomness properties compared to linear PNGs. To the best of our knowledge, this paper presents the first long-range correlation and distribution measurements in hardware for PNGs on FPGAs.

We describe an improved scheme for simulating Rayleigh fading channels which accurately reproduces the required channel statistics. The new scheme is based on the sum- of-sinusoids Rayleigh fading model because of its flexibility and efficient mapping onto hardware. Using numerical simulation it is shown that the statistical properties of the generated fading variates, such as the probability density function, the autocorrelation, and the level crossing rate, follow the desired theoretical properties. A fixed-point implementation of the fading channel simulator on a field-programmable gate array utilizes only 5% of the configurable slices and generates over 200 million 16-bit fading variates per second.

Multiple-input-multiple-output (MIMO) systems are capable of delivering substantially higher throughput under favorable circumstances, at the cost of complexity in terms of the number of parameters that must be estimated and processed. Our results are directed toward lowering this computational complexity through reduction of channel information required at the transmitter. Specifically, we examine the case when only the number of degrees of freedom supporting a threshold SNR is known at the transmitter. We show capacity is maximized in this case with a rank-reduced MIMO channel and an equal power distribution over all available antennas. The number of independent sequences to transmit, i.e. the amount of rank-reduction, is computed by defining the problem such that the optimal number of active data sequences is a capacity maximization problem solvable at the receiver whose results are used at the transmitter. It is shown that in the absence of channel state information, the received SNR for each potential sub-channel is sufficient to determine the optimal number of independent data sequences to transmit.

We demonstrate that receive antenna selection (RAS) provides significant increase in the achievable sum rates for multi-user MIMO wireless downlinks that employ block diagonalization (BD) to achieve orthogonal space division multiplexing (OSDM), where each user terminal has one or more antennas. Although dropping one or more receive antennas at a user terminal reduces its capacity and correspondingly, the system sum capacity, judicious RAS improves the projected channel spatial mode gains and provides additional degrees of freedom to all other terminals within the BD-OSDM context. In this way there is mutual benefit to be shared among users when RAS is applied to all users and numerical results show significant sum rate gains despite sum capacity loss due to RAS. In many cases, users with reduced array sizes also enjoy increased channel rates. When projected virtual channels are used as a means of spatial mode allocation, this RAS concept is also beneficial and may be referred to as spatial mode selection (SMS). RAS/SMS is therefore a necessary first step in any resource allocation and power control exercise for BD-OSDM. Further, the same RAS/SMS algorithms for sum rate maximization also provide a systematic means of resource allocation and power control. To avoid exhaustive RAS search, which has exponential complexity, efficient RAS algorithms with linear complexity and near optimal performance are proposed.

We consider two strategies for communication in ad hoc networks with local node cooperation. Groups of adjacent nodes transmit and relay information simultaneously in order to increase the throughput. For the proposed schemes we derive achievability bounds on the communication rates and demonstrate sizable gains over the corresponding time division multiple access based strategies. The obtained bounds closely follow recently derived information theoretic upper bounds on communication rates. Additionally we address the impact of traffic localization on the achievable rates and the receiver complexity.

High-speed wireless sensor networks are currently being considered for a variety of communication application such as environmental, medical, industrial or security scenarios. For increased transmission rates given the limited embedded battery lifetime, ultra-low-power circuitry is needed in the sensor and processors. Much research is being undertaken in these different areas at the device, circuit, system and network levels Although using error control coding (ECC) potentially reduce the required transmit power for reliable communication, higher decoder complexity increases the required processing energy. The above tradeoff is explored in this paper to find when use of ECC results in more power-efficient systems. Several recently implemented decoders are analyzed, comparing both analog and digital implementations. The four most energy efficient decoders are analog decoders. The best analog decoder becomes energy-efficient at about 1/4 the distance of the best digital implementation.

The paper presents an overview survey of emerging wireless technologies suitable for short reach (<100m) RF communication starting from the existing high bit-rate systems (802.11n, 802.15.3a) and ending with 60 GHz mm-wave radios. Basic principles, power dissipation levels and hardware realization challenges in silicon are discussed. Prospects of building technologies that achieve Gigabit per second data rates are investigated.

MIMO wireless downlinks using transmit zero-forcing beamforming (TZFBF) with MTtransmit antennas can serve up to K = MTreceivers, each equipped with one antenna. To maximize the sum rate, waterfilling can be used. It is shown that unlike classical waterfilling, TZFBF waterfilling cannot simply drop the poorer spatial modes during optimization. Instead, receive antenna selection (RAS) must be incorporated and achieving the maximum sum rate requires an exhaustive search over ∑|s|(MrC|s|) = 2MT−1 iterations to find the optimal subset S of active receivers where |s| = 1,..., MT. In principle, a separate RAS algorithm can be used in conjunction with waterfilling to reduce the exponential complexity O(2MT) of the exhaustive search to linear complexity O(MT). We develop optimization algorithms that emulate classical waterfilling by progressively reducing the effects of poor spatial modes in MTiterations. They do so by performing RAS jointly during the waterfilling process at little additional complexity. By avoiding a separate RAS process, complexity is thus further reduced. For the typical case where K>MT, we propose a 2-phase framework that helps reduce the overall complexity while meeting the TZFBF dimensional constraints. Numerical results over different channel conditions are given.

Traffic demands in networks play a vital role in determining the transmission rate that a network can support between communicating users. We derive information theoretic upper bounds on the rate per communicating source-destination pair in ad-hoc wireless networks with a non-uniform traffic pattern. The upper bounds are tight and closely follow the achievability bounds recently given by Tabet and Knopp. Furthermore, we show that in the case of large signal attenuation, the bounds hold even when the cooperation among the users is limited to certain region of the network domain

A serially concatenated coding system which can operate without channel state information (CSI) with use of a simple channel-estimation technique is presented. This channel-estimation technique uses the inner decoder's a posteriori probability (APP) information about the transmitted symbols to form a channel estimate for each symbol interval, and is termed "APP channel estimation." The serially concatenated code is comprised of an outer rate-2/3 binary error-control code, separated by a bit interleaver from an inner code consisting of an 8-phase-shift keying (PSK) bit mapping and differential 8-PSK modulation. Coherent decoding provides bit-error rate performance 0.6 dB from 8-PSK capacity for large interleaver sizes. APP channel-estimation decoding without initial CSI over constant and random walk phase models shows near-coherent results, with fractions of a decibel performance loss for random walk and linear phase models.

Iterative interference cancellation of coded code-division multiple access (CDMA) using random spreading with linear cancellation is analyzed. If users are grouped into power classes and Shannon bound approaching codes are used, a geometric power distribution achieves the additive white Gaussian noise (AWGN) channel Shannon bound as the numbers of classes becomes large. The optimal distribution of the size of these classes is shown to be uniform. If users are grouped into different rate classes with equal powers among equal rate users, the Shannon bound for AWGN channels can be achieved with an arbitrary distribution of the classes sizes, provided that the size of the largest rate class obeys the mild condition that its ratio of size to processing gain is much smaller than the inverse of the signal-to-noise ratio (SNR). The case of equal powers and equal rates among all users is addressed as a "worst case" scenario. It is argued that simple repetition codes provide for a larger achievable capacity than stronger codes. It is shown that this capacity monotonically increases as the rate of the code decreases. A density evolution analysis is used to show that the achievable rates exceed those of a minimum-mean square error filter applied to the uncoded signals. This lower bound is tight for small ratios of bit energy to noise power, and otherwise the iterative cancellation receiver provides an appreciably larger capacity. Relating to recent result from the application of statistical mechanics it is shown that the repetition-coded system with iterative cancellation achieves the performance of an equivalent optimal joint detector for uncoded transmission

Random packet CDMA, a novel packet-based multiple access scheme for connectionless, uncoordinated random channel access is proposed. Random packet CDMA, or RP-CDMA, utilizes a novel packet format which consists of a short header and a data portion. Each header is spread with a unique spreading code which is identical for all users and packets, while the data portion of each packet is spread by a randomly chosen spreading sequence. The receiver operates in two stages: header detection and data detection. For header detection a conventional spread spectrum receiver is sufficient. Headers are spread with a large enough processing gain to allow detection even in severe interference. The data portion is decoded with a sophisticated receiver, such as a multiuser detector, which allows for successful decoding of overlapping active packets. It is shown that the RP-CDMA system is detector capability limited and that it can significantly outperform spread ALOHA systems whose performance is limited by the channel collision mechanism. RP-CDMA also experiences a much smaller packet retransmission rate than conventional or spread ALOHA, and provides better spectral efficiencies

This paper examines ways to recoup the performance loss incurred when using the min-sum approximation instead of the exact sum-product algorithm for decoding low-density parity check codes (LDPCs). Approximations to the correction factor exactly expressing the difference between these two decoding algorithms exist for degree 3 check nodes, and can be applied to higher degree nodes by subdividing them into component degree 3 nodes. However, this results in replication of the approximation. An asymptotic expression for the correction factor at a check node of any degree is derived in this paper, and used to develop two simple approximations to the correction factor, matched to the check node degree. One has very low complexity, and both only need be applied once per check node extrinsic message. Simulation results are presented for each check node approximation when decoding a regular and an irregular LDPC. Both degree-matched check node approximations achieve sum-product decoding performance

This paper presents a stochastic algorithm for iterative error control decoding. We show that the stochastic decoding algorithm is an approximation of the sum-product algorithm. When the code's factor graph is a tree, as with trellises, the algorithm approaches maximum a-posteriori decoding. We also demonstrate a stochastic approximations to the alternative update rule successive relaxation. Stochastic decoders have very simple digital implementations which have almost no RAM requirements. We present example stochastic decoders for a trellis-based Hamming code, and for a block turbo code constructed from Hamming codes

This paper introduces interleaver constraints to eliminate low squared Euclidean distance (SED) error events for a specific serially concatenated code (SCC). These interleaver constraints eliminate specific multiple error events as well as single error events. Factor graph representations of the interleaver allows to view multiple error events as short cycles in a graph. Interleaver construction rules which eliminate short cycles for an interleaver graph representation also eliminate the minimum SED error events for this SCC.

All analog circuits are affected by device mismatch, slight errors in the physical characteristics of analog devices. In many applications, mismatch reduces precision, making power and complexity advantages irrelevant. In this paper, we examine the effects of mismatch for decoders based on Gilbert multipliers and the sum-product algorithm.

In order to achieve high capacity packet transmissions in wireless ad hoc networks, the system throughput for a connectionless, distributed channel accessing scheme exploiting joint detection receivers is proposed and analysed. Joint detection allows many concurrent, asynchronous packet transmissions to occur, thereby enhancing the capacity of a system. A novel packet format with physical layer header's containing an additional code identifier (CID) field as proposed by P. Gupta and P. R. Kumar (March 2000) is employed and CSMA is used for the physical layer header (PLH) of the packet. The data portion of each packet is spread by a unique random binary signature sequence identified by the code identifier field (CID) which allows joint detection of the asynchronous, overlapping packets. A comparison of using CSMA and Aloha multiple access for the PLH is made for a bimodal packet length distribution typical in TCP traffic. Further, an upper hound on the throughput capacity considering the proposed channel accessing schemes is given indicating a significant improvement of system and network throughput is possible with respect to conventional methods.

A simple serially concatenated turbo code using differential 8PSK encoding as the inner code and a [3,2,2] parity code as the outer code is studied. This system is decoded according to turbo principles with iterative exchange of extrinsic probabilities, without differential demodulation. Results 0.6 dB from 8-PSK capacity are demonstrated for large interleaver size. Decoding over channels without prior synchronization is demonstrated to be feasible even with significant phase offset and phase noise, using a simple channel estimator that utilizes the extrinsic output symbol probabilities from the differential APP decoder.

The combination of forward error control coding (FEC) with code division multiple access (CDMA) using random spreading sequences is considered. Through tracking the input-output variance evolutions of the decoding components, which are the CDMA interference resolution function and the individual error control decoders, an analysis of the iterative joint detection was previously presented. It was shown this variance transfer (VT) analysis between component decoding devices gives very accurate description of the convergence properties of the iterative joint detector for a CDMA system with all users received at equal energy. In this paper, the VT analysis is extended to the more realistic cases where the different users have unequal power levels. It is shown that the effective VT functions modified w.r.t. the unequal power scenario can be used to analyze the iterative reception employing a wide variety of linear filters for interference resolution, in a manner very similar to that in the equal power case. Numerical simulations verify the results predicted by VT analysis.

Space-time block codes based on orthogonal designs, known as space-time orthogonal block (STOB) codes, have recently been proposed. It has been noted that when the number of transmit antennas is more than two, full-rate STOB codes could only be designed for real-valued data symbols. Real-valued symbols form pulse amplitude modulated (PAM) sequences whose bandwidth efficient transmission is only possible through single side-band (SSB) modulation. We proposed cosine modulated filter bank (CMFB) multicarrier modulation (MCM) as a bandwidth efficient method of implementing full-rate STOB-coded systems. The impact of channel distortion on the received signal is studied, and a method of designing a zero-forcing equalizer that removes intersymbol interference (ISI) and interchannel interference (ICI) in the system is developed. Some relevant properties of the proposed system are also reported.

A new packet-based, multiple access scheme for connectionless, uncoordinated random access is proposed using code-division multiple access (CDMA) as the typical access method. The new method uses a novel packet format with a common header with identical spreading codes for the data portion. The receiver operates in two stages: header detection using a conventional spread spectrum receiver and data detection using a multiuser detector to allow for decoding of overlapping active packets. The headers are spread with a large enough processing gain to allow detection even in severe interference. It is shown that this system is detector capability limited and that it can significantly outperform conventional ALOHA systems whose performance is limited by the collision mechanism. This system also experiences a much smaller packet retransmission rate and better spectral efficiency than conventional or spread ALOHA.

Low complexity multistage filters are inserted into the soft-information exchange paths of an iterative joint detector for error control coded CDMA signals. It is shown that these filters approach the performance of the optimal, but complex, per-user minimum-mean square error filter with a few stages. A variance transfer analysis is presented to quantify the performance of this method and to study the case of unequal received power distributions.

Serial concatenation of simple error control codes and differential space-time modulation is considered. Decoding is performed iteratively by passing symbol-wise a posteriori probability values between the decoders of the inner space-time code and the outer code. An extrinsic information transfer analysis is used to predict thresholds for outer convolutional codes of various memory orders and a simple outer parity-check code. This parity-check code is well matched to the inner differential space-time code and achieves a bit-error rate (BER) of 10/sup -6/ less than 2 dB from the Shannon capacity of the fast fading multiple antenna channel. The differential space-time code can also be used to generate a priori information in the absence of channel knowledge. This information can be exploited by a channel estimator inserted into the decoding iteration. It is demonstrated that the inner space-time code provides soft training symbols from periodically inserted training symbols. The reliability of these soft training symbols does not depend on the speed of the channel variations, but on the structure of the inner code and the signal-to-noise ratio (SNR). Simulation studies confirm these findings and show that the proposed system with no initial channel knowledge achieves a performance very close to that of the system with perfect channel knowledge.

A code-division multiple-access (CDMA) system in conjunction with serial concatenation of FEC codes is considered. Iterative decoding strategies are used at the receiver. The component decoders are functionally described by variance transfer (VTR) charts. Pinch-off SNRs, where the iterative decoding starts to function, are precisely predicted by this approach. Designs of the multiuser detection (MUD) method and the component FEC codes have been carried out and system performance to within about 1 dB of the Shannon capacity of the multiple-access channel is demonstrated.

This paper deals with the fusion of random variables when cross covariances are unknown. This is a vital problem in nearly every real world application since cross covariances are often impossible to obtain, but also cannot be ignored. We provide a rigorous derivation of the fusion equations which are also known as covariance intersection. This approach allows one to derive an iterative scheme for simultaneous mapping and localization. The algorithm can also be used for multi-robot explorations where highly correlated decentralized maps have to be fused to form a consistent global map. We show the mapping and localization results based on dense laser range scans.

The serial concatenation of standard convolutional codes with differential space-time modulation is considered for fast flat fading multiple antenna channels. Decoding is performed iteratively by passing symbol-wise a-posteriori probability information between the decoders of the inner space-time code and the outer convolutional code. An input-output extrinsic information transfer analysis is used to predict thresholds for outer codes or various memory orders. Simulation results show that this system can achieve bit error rates below 10/sup -4/ at less than 2.5 dB from the Shannon capacity or the multiple antenna channel.

Serial concatenation of standard convolutional or block codes with differential space-time modulation is considered for flat fading multiple antenna channels. Extrinsic information transfer is used to predict thresholds for various outer codes. Using the differential structure of the inner code near coherent performance is obtained without the use of training symbols.

Joint iterative decoding of forward error control (FEC) encoded code-division multiple-access (CDMA) systems is studied. Serially concatenated convolutional codes are used as FEC codes. We apply iterative decoding to this system and study the effect of component decoders and different decoding partition schemes via a variance transfer analysis. It is shown that distinct partition schemes result in similar pinch off SNRs.

A design approach is presented for soft-decision decoding of block product codes ("block turbo codes") using analog computation with MOS devices. Application of analog decoding to large code sizes is also considered with the introduction of serial analog interfaces and pipeline schedules.

Joint iterative decoding of multiple forward error control (FEC) encoded data streams is studied for linear multiple access channels, such as code-division multiple access (CDMA). It is shown that such systems can be viewed as serially concatenated coding systems, and that iterative soft-decision decoding can be performed successfully To improve power efficiency, powerful FEC codes are used. These FEC codes are themselves serially concatenated. The overall transmission system can be viewed as the concatenation of two error control codes with the linear multiple access channel, and soft-decision decoders are used at each stage. A variance transfer function approach applied to the analysis of this system captures the role of the component decoders in an overall iterative decoding system. We show that this approach forms a methodology to study the effects of the component codes as well as that of the iteration schedule. Analysis and simulation examples are presented for transmission systems that operate close to the Shannon limit and illustrate the accuracy of the analysis.

We consider the convergence in norm of several iterative implementations of linear multiuser receivers, under the assumption of long random spreading sequences. We find that asymptotically, linear parallel interference cancellation diverges for systems loads of greater than about 17%. Using known results from the theory of iterative solutions for linear systems we derive optimal or near-optimal relaxation parameters for parallel (first- and second-order stationary, Chebyshev) and serial cancellation (successive relaxation) methods. An analytic comparison of the asymptotic convergence factor for the various methods is given. Simulations are used to verify results for finite size systems.

Multi-user detection of CDMA signals is studied in the light of iterative processing. The factor graph representation of a CDMA system is used to develop a low-complexity joint detection algorithm. This algorithm replaces the large sums that appear at the multiple access nodes by simple averages and ignores the error control code network of the interfering users, assuming a fixed probability distribution on the output symbols of the interfering users. This is optimal in the case where the code network for the interfering users is unknown, and amounts to an error control decoder using a multi-user metric generator. Simplifications of the multi-user metric generation lead to linear iterative metric generators. Simulation and analytical results on the performance of these detectors are shown and illustrate complexity/performance trade-offs.

It is demonstrated that optimal multiple-user detection in a linear multiple-access system with identical crosscorrelations requires only /spl Oscr/(KlogK) operations instead of the worst case O(2/sup K/). A simple optimal detection algorithm is given, which when generalized has a complexity that is exponential not in the number of users but in the number of unique crosscorrelation values.

This paper presents a blind adaptive CDMA receiver that requires no knowledge of the spreading codes, the delays, and the energy of the received signals associated with the interfering users. Our receiver is based on linear interference cancellation and adaptive interference signal subspace tracking. It has error control coding embedded in the detector structure and employs "branch processing" to detect the bit stream. Simulation results demonstrating that the receiver suffers negligible performance loss over systems with complete knowledge of the interfering users are presented.

Presents the software framework SMARTSOFT to implement sensorimotor systems. SMARTSOFT not only contains software components to support a modularized implementation but also structural rules and templates. These ensure that the implementation of modules is conforming with an overall multilayer system architecture. Since the provided structures allow the exact specification of the external behavior of modules in terms of interfaces and dependencies, the interaction with a symbolic task execution layer is explicitly supported. SMARTSOFT significantly eases the implementation and integration of new modules into a complex sensorimotor system which for example provides the opportunity even to compare and reuse different modules on a mobile platform. SMARTSOFT has already proven its usefulness within the collaborative research center (SFB 527) "Integration of Symbolic and Subsymbolic Information Processing in Adaptive Sensorimotor Systems".

We view the asynchronous random code division multiple-access (CDMA) channel as a time-varying convolutional code. We study the case where the users encode their data, and, therefore, the single user transmitters and the CDMA channel appear as the concatenation of two coding systems. At the receiver we employ serial turbo decoding strategies. Unlike conventional turbo codes where both the inner and outer code may be selected, in our case, the inner code is due to the CDMA channel which we assume to be random. Nevertheless, the decoding system resembles the decoder of a serial turbo code and single-user performance is obtained even for numbers of users approaching the spreading code length.

We address the problem of detecting and tracking people with a mobile robot. The need for following a person with a mobile robot arises in many different service robotic applications. The main problems of this task are realtime-constraints, a changing background, varying illumination conditions and a non-rigid shape of the person to be tracked. The presented system has been tested extensively on a mobile robot in our everyday office environment.

This paper presents an efficient approach for reactive collision avoidance taking into account both vehicle dynamics and nonholonomic constraints of a mobile robot. Motion commands are generated by searching the space of actuating variables. Vehicle dynamics are considered by restricting the search space to values which are reachable within the next time step. The final selection among admissible configurations is done by an objective function which trades off speed, goal-directedness and remaining distance until an obstacle is hit when moving along the chosen path. The presented approach differs from previous ones in the selective use of precalculated lookup tables. These are the key to efficiency, and they especially allow the use of any-shaped robot contours. Furthermore, obstacle information from different sources can easily be considered without preprocessing. Extensive experiments on different robots have shown robust operation in dynamic and unprepared indoor environments with speed up to 1 m/s.

This paper introduces an iterative multiuser receiver for direct sequence code-division multiple access (DS-CDMA) with forward error control (FEC) coding. The receiver is derived from the maximum a posteriori (MAP) criterion for the joint received signal, but uses only single-user decoders. Iterations of the system are used to improve performance, with dramatic effects. Single-user turbo code decoders are utilized as the FEC system and a complexity study is presented. Simulation results show that the performance approaches single-user performance even for moderate signal-to-noise ratios.

In this paper, receiver design and performance analysis for coded asynchronous code-division multiple access (CDMA) systems is considered. The receiver front-end consists of the near-far resistant multiuser detector known as the projection receiver (PR). The PR performs multiple-access interference resolution and is followed by error-control decoding. The output of the projection receiver yields the appropriate metric (i.e., soft information) for decoding of the coded sequences. An expression for the metric is derived that allows the use of a standard sequence decoder (e.g., Viterbi algorithm, M-algorithm) for the error-control code. It is then shown that the metric computer has an elegant adaptive implementation based on an extension of the familiar recursive least squares (RLS) algorithm. The adaptive PR operates on a single sample per chip and achieves a performance virtually identical to the algebraic PR, but with significantly less complexity. The receiver performance is studied for CDMA systems with fixed and random spreading sequences, and theoretical performance degradations with regard to the single-user bound are derived. The near-far resistance of the PR is also proven, and demonstrated by simulation.

This paper discusses a code-division multiple access (CDMA) iterative multi-user receiver with forward error control (FEC) decoding. The maximum a-posteriori probability (MAP) criteria is used to derive the receiver. The decoding is done using a turbo-code decoder with modifications which are discussed. Iterations of the system are used to attain large performance improvements over conventional systems.

We consider a CDMA system with error-control coding. Optimal joint decoding is prohibitively complex. Instead, we propose a sequential approach for handling multiple-access interference and error-control decoding. Error-control decoding is implemented via single-user soft-input decoders utilizing metrics generated by linear algebraic multiuser metric generators. The decorrelator, and a new scheme termed the projection receiver, are utilized as metric generators. For a synchronous system, the coded performance of the projection receiver metric is shown to be superior to the decorrelator even though they are equally complex. Also, the theoretical degradation relative to the single user bound is derived.

A simple method to compute the minimum distance D/sub min/ in multiuser code division multiple access (CDMA) systems based on the Cholesky decomposition of the positive-definite symmetric correlation matrix is proposed. Although finding D/sub min/ is known to be NP-hard, this decomposition allows the computation of the minimum distance and the asymptotic efficiency of optimum multiuser detection to be performed very efficiently in almost all cases of practical interest. Numerical results for synchronous CDMA with binary random signature waveforms of length 31 are used to illustrate the method.

A collision-type multiple access system is investigated in which every user transmits symbols from a common N-ary frequency-shift-keyed signal alphabet. We present a series of information-theoretic properties of the associated mathematical channel model. In the absence of noise, we calculate a large system approximation to the sum capacity, which is used to show that, in the limit, the combining of the multiple-accessing and coding results in no loss in capacity, compared to a fixed-allocation scheme. The presence of thermal noise or undetected users influences the capacity. Bounds on the capacity of the channel in the presence of thermal noise are calculated, as well as the capacity of the system in the presence of interfering users. Finally, a new iterative multiuser detector, the consensus decoder, is described and simulation performance results are shown. It is demonstrated that this decoder can operate to within approximately 70% of the channel capacity.

A new multiuser receiver for synchronous code-division multiple-access (CDMA) systems with error control coding is proposed. The receiver achieves interference cancellation by projecting the undesired users onto the space spanned by the desired users' signal vectors. The detector calculates the least squares (LS) estimate of the interfering users data, that is used to yield an adjusted metric for maximum likelihood sequence estimation (MLSE) for the desired users' sequences. Simulation results indicate that close to optimal performance can be achieved when all but one of the users are projected using only a single user decoder for the desired user. Further, an adaptive receiver structure based on the recursive LS update is presented that is well-suited for DSP implementation due to it's computational efficiency.

The error probability of wide-band signals on multibeam Rayleigh fading channels is calculated in closed form using similarity transformations of Gaussian quadratic forms. It is shown how this approach can be used to calculate the error probability on arbitrary complex Gaussian channels, even when the beams are correlated. As an application example the author considers a wide-band spread-spectrum system using code-division multiple access (CDMA).

The performance of multi-user orthogonal frequency division multiplexing (OFDM) with raised cosine time domain pulse shaping is studied on the satellite mobile channel (SMC) and the two-path Rayleigh fading channel. A multi-user time and carrier frequency synchronization scheme is discussed. The relationship between the synchronization requirement and the roll-off rate (/spl beta/) of the pulse shaping is explored. In Gaussian and typical Rician (K=10) channels, multi-user OFDM with /spl beta/=0.2 requires timing and carrier frequency synchronization accuracies of 4% and 2%, respectively, in order to avoid severe degradation due to multi-user adjacent channel interference, while on the two-path Rayleigh channel these values must be better than 2% for error floors less than 10/sup -3/. Timing and carrier frequency accuracies are normalized to T and 1/T respectively, where T is the symbol duration. Multi-user OFDM is less affected by the multipath time delay and slightly more severely affected by the Doppler frequency shift than traditional FDM with root Nyquist filtering (FDM/RNF) for the same value of /spl beta/.>

Commentson a paper by Rouanne and Costello Jr. (IEEE Trans. Inform. Theory, vol.34, p.1011-20, 1988). It is shown that the derivation in the above paper is incorrect and hence the lower bound obtained is not valid for general trellis-coded modulation schemes. A corrected bound is presented to remedy this flaw. Also shown is a similar lower bound derived from the average distance structure. Both bounds have similar asymptotic behavior and are linear in the constraint length. Unfortunately, they are bad and not very useful for short and medium constraint lengths. The present author points out that there is still much room for further research in finding a good lower bound on the maximal minimum distance for trellis-coded modulation schemes.>

In the autonomous mobile systems project (AMOS), the FAW uses a mobile robot to study questions related to the deep integration of sub-symbolic and symbolic information processing. AMOS aims at methods for autonomously acquiring new concepts via induction from its interaction with its environment. This paper presents an architecture which integrates both symbolic planning as well as nonsymbolic reactive mechanisms, thus providing a basic autonomy, so that the robot can freely maneuver around without any detailed model of itself and its complex real-world environment . Substantial differences between expectation and observation are used as hints-generated via the robot's interaction with the environment-to situations which are of relevance for the robot. In particular, the concepts of plan breakdown and region of interest play a fundamental role. Autonomously, based on the robot's decision, images are taken and clustered without supervision into groups which are expected to correspond to semantically similar situations. These hypotheses shall be used in further work as a necessary pre-requisite in order to autonomously generate a new concept relating the recognition of such perception classes to appropriate actions.>

A general analysis of trellis coded modulation (TCM) on time-selective fading channels with correlated signal samples is presented. Gaussian quadratic forms are used to evaluate the pairwise sequence error probability. The effect of interleaving is studied and it is shown that moderate interleaving with respect to the doppler frequency of the channel suffices to achieve good performance. The analysis is then extended to treat non-ideal estimation of the complex channel gain in the coherent case using a general linear channel estimator. It is shown that differential phase shift keying (DPSK) and differential trellis coded modulation (DTCM) on correlated fading channels call be treated analytically and that the error probability of DTCM, like of coherent TCM, depends only on the Euclidean distance of the error sequences. Results show that DTCM suffers a degradation of up to 3 dB with respect to coherent TCM on slow fading channels but performs better on fast fading channels. Error floor curves are presented for DTCM for different interleaving depths. The event error probability P/sub e/ and the bit error probability P/sub b/ of TCM, approximated by truncated averaged union bounds, are calculated for the codes used. A comparison of the approximations of P/sub b/ with simulation results show that the union bound is relatively loose for fading channels.>

A general class of trellis codes whose generating trellis can be labeled by a set of linear labels is defined. It is shown that for these codes there exist conditions on the signal mapping that make it possible to calculate the distance spectrum with respect to a distance measure (e.g., Euclidean distance) assuming an arbitrary correct sequence. This class of new codes contains most known trellis codes. It is then shown that this approach can be extended to calculate the distance spectra of certain trellis codes on intersymbol interference channels, e.g., Ungerboeck 8-PSK trellis codes. Examples of the distance spectra and union bounds on the bit-error probability for channels with and without intersymbol interference are evaluated. A comparison to simulation results is used to demonstrate the tightness of the bounds.>

A dual-mode burst-error-correcting algorithm that combines maximum-likelihood decoding with a burst detection scheme is presented. The decoder nominally operates as a Viterbi decoder and switches to time diversity error recovery whenever an uncorrectable error pattern is identified. It is demonstrated that the new scheme outperforms interleaving strategies under the constraint of a fixed overall decoding delay. It also proves to be more powerful than known adaptive burst decoding schemes, such as the Gallager burst finding scheme. As the new method can be used with soft decision decoding, it is mainly intended for use on random-error channels affected by occasional severe bursts.>

The authors apply a general method of bounding the event error probability of TCM (trellis-coded modulation) schemes to fading channels and use the effective length and the minimum-squared-product distance to replace the minimum-free-squared-Euclidean distance as code design parameters for Rayleigh and Rician fading channels with a substantial multipath component. They present 8-PSK (phase-shift-keying) trellis codes specifically constructed for fading channels that outperform equivalent codes designed for the AWGN (additive white Gaussian noise) channel when v>or=5. For quasiregular trellis codes there exists an efficient algorithm for evaluating event error probability, and numerical results which demonstrate the importance of the effective length as a code design parameter for fading channels with or without side information have been obtained. This is consistent with the case for binary signaling, where the Hamming distance remains the best code design parameter for fading channels. The authors show that the use of Reed-Solomon block codes with expanded signal sets becomes interesting only for large value of E/sub s//N/sub 0/, where they begin to outperform trellis codes.>